'Sherlock' Season 4 Photos

From left: Amanda Abbington (Mary), Benedict Cumberbatch (Sherlock) and Martin Freeman (John Watson) in Sherlock Season 4, premiering January 1, 2017 on Masterpiece on PBS.

From left: Mark Gatiss (Mycroft Holmes) and Benedict Cumberbatch (Sherlock) in Sherlock Season 4, premiering January 1, 2017 on Masterpiece on PBS.

Martin Freeman (John Watson) in Sherlock Season 4, premiering January 1, 2017 on Masterpiece on PBS.

Toby Jones (Culverton Smith) in Sherlock Season 4, premiering January 1, 2017 on Masterpiece on PBS.

Mark Gatiss (Mycroft Holmes) in Sherlock Season 4, premiering January 1, 2017 on Masterpiece on PBS.

Una Stubbs (Mrs. Hudson) in Sherlock Season 4, premiering January 1, 2017 on Masterpiece on PBS.

From left: Martin Freeman (John Watson), Amanda Abbington (Mary) and Benedict Cumberbatch (Sherlock) in a behind-the-scenes shot from Sherlock Season 4, premiering January 1, 2017 on Masterpiece on PBS.